Transcript MEMBRANE PERMEABILITY

MEMBRANE PERMEABILITY
PERMEABILITY
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The membrane must allow water molecules
to diffuse through. It is permeable to water.
If a concentrated solution is separated from a
dilute solution by a suitable membrane, water
will pass from the dilute to the concentrated
solution.
In fact, water passes both ways but faster
from the dilute to the concentrated solution.
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The membrane allows only molecules of a certain size to
diffuse through it, it is called selectively permeable.
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The cell membrane functions as a selectively permeable
membrane.
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The cell sap and cytoplasm function as fairly
concentrated solutions.
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In a plant cell, the cell membrane acts as a
selectively permeable membrane
The cell wall is freely permeable to water
The vacuole contains a solution of salts and sugars
If there is water outside the cell, it will diffuse by
osmosis into the vacuole
The vacuole will expand, pushing the cytoplasm
outwards against the cell wall
Diffusion
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Most common type of passive transport.
Diffusion – is the random movement of particles (atoms,
ions, molecules) from a region of high concentration to
low concentration, down a concentration gradient.
Molecules diffuse down a concentration gradient.
Diffusion stops when molecules dispersed evenly (with
no concentration gradient), and a state of equilibrium is
reached.
Process of diffusion
Dissolved substance diffuse throughout liquid in which
they are dissolved.
Why is diffusion important?
Diffusion is important for:
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Gaseous exchange (oxygen, carbon dioxide) during respiration
and photosynthesis
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Excreting waste products e.g. ammonia, water, mineral salts
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Absorption of digested food into blood through walls of small
intestine.
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Enables animals to detect food by smell.
Osmosis
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A form of passive transport process
Osmosis – diffusion/movement of water molecules across
a selectively permeable membrane from a region of higher
water concentration to a region of lower water
concentration.
A partially/selectively permeable membrane only allows
certain molecules to pass through it but not others.
What is the difference between
diffusion and osmosis?
Osmosis is the net movement of
water molecules across a
Partially-permeable membrane.
Water molecules move randomly
with a certain amount of kinetic
energy…
Distilled water separated by a partiallypermeable membrane:
Water molecules are moving from
one side of the membrane to the
other but there is no net osmosis
If a substance is dissolved in water, the
kinetic energy of the water molecules is
lowered.
This is because some water molecules
aggregate on the surfaces of the other
molecules…
Osmosis demonstration
The osmotic potential of a cell
is known as its WATER
POTENTIAL. For animal cells,
the water potential is the
osmotic potential of the
cytoplasm.
Osmosis and Plant cells
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In plant cells, cell sap contains dissolved salts
and sugar. If cell sap has lower water potential
than that of surrounding solution, water enters
by osmosis.
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Plant cell will swell and become firm / turgid.
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Plant cell walls prevent cells from bursting.
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Turgor pressure - outward pressure which cell
sap exerts against inside wall of cell.
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Turgor helps to support soft tissues in plants
Osmosis and Plant cells
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If cell sap has higher water potential than
surrounding solution, water moves out of
the vacuole and cytoplasm shrinks away
from the cell wall.
Cell loses its turgor, shrinks and
becomes flaccid or soft. The cell becomes
plasmolysed.
Plasmolysis - shrinkage of cytoplasm
away from the cell wall when plant cells
are immersed in a solution of low water
potential.
Plasmolysis causes land plants to wilt, in
non-woody parts of plants e.g. leaves,
shoots
Dilute vs Concentrated solutions
A
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B
Hypotonic - Dilute solution A ( higher water potential)
compared to concentrated sugar solution B ( lower
water potential)
Hypertonic - Solution B has water potential compared
to solution A
Isotonic - when both solutions have the same water
potential (‘iso’: same as; ‘tonicity’: strength of
solution).
Osmosis in plant and animal cells
Osmosis and Plant cells
Plasmolysis
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Plasmolysis is the loss of water from the cell
by osmosis,and this is evident when the cell
contents pull away from the rigid cell wall as
the water moves out.
Steps of Plasmolysis
Wilting due to loss of water!!
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